We are writing in order to reference and acknowledge the following research article on the phoshoproteome of the archaeon Haloferax volcanii, which was accidentally “overlooked’’ in our research article in PLoS ONE:

Kirkland, P. A., M. A. Gil, I. M. Karadzic and J. A. Maupin-Furlow. 2008. Genetic and proteomic analyses of a proteasome-activating nucleotidase A mutant of the haloarchaeon Haloferax volcanii. J. Bacteriol. 190:193-205.

In this work the authors, by employing a comparative proteomics approach based on phosphospecific fluorescent staining of proteins separated by two-dimensional gel electrophoresis, showed a 2.1-fold increase in the number of detected phosphorylated proteins when the proteasome-activating nucleotidase A (panA) gene was disrupted in the haloarchaeon Haloferax volcanii. In addition, the phosphoproteins of these strains were enriched by immobilized metal ion and metal oxide affinity chromatography (in parallel and sequentially) followed by tandem mass spectrometry to identify key differences in their proteomes as well as to add to the restricted numbers of known phosphoproteins within the Archaea. In total, 625 proteins (approximately 15% of the deduced proteome), and 9 phosphosites on 8 phosphorylated proteins were identified by these approaches. 31% (195) of the proteins were identified by multiple phosphoanalytical methods. Only one of the identified phosphosites, T533 of HVO_0806 Pyruvate kinase, was found in our work to be phosphorylated in the homologous Halobacterium protein OE1495R, pyruvate kinase, pykA, at T536. The authors also showed the identification of the first phosphotyrosine site in an archaeon: Tyr117 of the DNA-directed RNA polymerase subunit A’. Even though this is one additional piece of evidence that Tyr phosphorylation occurs in Archaea similarly to Eukarya, our experience is that this identification has to be confirmed with further experiments, and a better spectrum with higher sequence coverage and intensity of the identified fragments. It has to be noted that the sole phosphotyrosine-containing peptide identified in our dataset measured with 3.6 ppm mass deviation, was an outlier, therefore additional experiments will be needed to confirm the presence of tyrosine phosphorylation on this protein.

The 8 proteins described in this study were not mentioned in our publication in PLoS ONE.